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Gas-Phase Biofiltration 433

of the chemical on the microbial flora of the biofilter. Some highly water-soluble com-
pounds, such as ethanol, may pose problems if introduced in too high a concentration;
that is, the rate of solubility into the biofilm is greater than the rate of biodegradation, caus-
ing an accumulation in the biofilm and a toxic effect on the microbes (25). This toxic
effect then causes a decrease in performance and a degradation of the microbial flora in
the system. However, this can be addressed in some cases by using preacclimated highly
tolerant microbial species.
Acidity may build up in the medium as a result of the oxidation of compounds con-
taining sulfide, chloride, and so forth, which will yield an inorganic acid. These may be
removed by water flushing at regular intervals or by using a buffering agent such as
sodium hydroxide, magnesium hydroxide, calcium hydroxide, and so forth.
3.5. Comparison to Competing Technologies
As can be seen in Table 6, the odor-control techniques can be broken down into two
broad categories: (1) physical/chemical: adsorption, absorption, and catalytic com-
bustion; and (2) biochemical: biofiltration and bioscrubbing. When deciding on an
odor-control strategy, a number of factors must be considered. These factors include
flow rates, type and concentration of malodorous compounds, level of particulate
matter, and stability of flows and concentrations. A decision also can be made based
on comparing the lifetime costs of various treatment processes. As indicated earlier,
biofiltration is an established technique offering the advantages of high efficiency with
generally low operational and capital costs. The technology is based on utilization of
immobilized bacteria or fungi in a conventional packed-bed reactor. The operation
relies on absorption of the vapor-phase pollutant into a wet biofilm surrounding the
solid media. Subsequently, biocatalytic oxidation takes place by means of the immobi-
lized microbial species.
4. DESIGN CONSIDERATIONS/PARAMETERS

4.1. Predesign
It is important first to examine the pollutant gas to be treated. Important parameters
that need to be assessed are the compounds that are present in the gas stream and their
concentrations. Second the volumetric or mass flow rate and temperature of the gas
stream to be treated is required. Ideally, it is of great use in the design process if one can
obtain a history or a quantitative prediction of how these variables will vary, both tem-
porally and particularly for the constituents, how much the relative concentrations will
vary, and if any other compounds are likely to be present. If at all possible, it is ideal,
if a bench-scale and/or a pilot-scale study could be undertaken, to obtain a relationship
between the volumetric pollutant loading (usually expressed as g/m 3 /h) and the bed
gas
elimination capacity (EC, expressed in g/m 3 /h). A balance is required between the
gas
EC and the actual amount of pollutant removed. Often regulations state that a certain
percentage of pollutant must be removed rather than an actual EC.
From the point of view of mineralization of the pollutant, the kinetics of such a
process are likely to follow an inhibition-type model form. These types of model are
unstructured kinetic model generally developed, or extended, from the Monod equa-
tion for substrate uptake (e.g., Haldane/Andrews, Levenspiel). The influence of the